There are many duct-like organs such as a blood vessel, a biliary tract, a urinary tract, a digestive tract, and the like. These duct-like organs may be caused to have characteristic diseases such as a stricture, occlusion, dilation, or the like due to respective various causes. For example, the blood vessel may be caused to have stenosed diseases such as a stricture and occlusion or dilative diseases such as an aneurysm of the aorta and a varix.
Particularly, the aneurysm of the aorta refers to the abnormal dilation of weakened arterial wall caused by hardening or inflammation of the main artery, and it is a critical disease wherein the arterial wall, if left untreated, is gradually dilated from the pressure of a blood flow, and the swelling wall can no longer resist the blood pressure to burst. It therefore requires an immediate treatment or procedure for preventing the bursting and bleeding.
In recent years, a tubular device made of a metal called a stent is often used when an abnormal stenosed part or dilated part of a blood vessel is treated, since no excess invasion such as a surgical operation is required. For example, when an aneurysm of the aorta is treated, the treatment is conducted in such a manner that a stent graft, a device, obtained by covering the above mentioned stent with a tubular member made of a synthetic resin, is used as an artificial blood vessel, and the stent graft is placed within the aneurysm to allow the blood to flow inside the artificial blood vessel, so that the blood pressure is not exerted directly on the swelling wall.
As a stent, conventionally, some types of stents have been provided. Of these, there is used a tubular device obtained by bending a linear material (wire) made of a metal typified by stainless steel to form zigzag patterns, forming the top portion thereof into the form of a curve or curved line, connecting both ends of the linear material to form it into an annular unit, arranging such annular units in series and connecting them with connection struts. It is also another practice to constitute a tubular device from a metal mesh.
For example, when the stent is anchored or placed in a treatment site (diseased part), the stent or stent graft generally inserted in a sheath is restrained, diameter-contracted and loaded into a delivery kit such as a pipe-like cylindrically formed catheter through the inside of which a guide wire is placed. It is introduced into a blood vessel from an incision of a peripheral artery of joint of a leg along the guide wire and delivered to a treatment site such as an aneurysm of the aorta, and when it reaches the treatment site, the above delivery kit is released to remove the contraction of the stent or the like. The stent released from the contraction expands in diameter by itself, and in this state, it is placed or left in a diseased part inside the blood vessel thereby to protect the blood vessel. However, it has been found that the above conventional stent or stent graft (to be sometimes simply referred to as “stent or the like” hereinafter) has the following problems.
(1) The above mentioned stent or the like is basically formed of a tubular body made of a linear metal framework before it is inserted in a sheath. The stent or the like of the above type is to some degree improved in conformity after inserted in a diseased part, by means of design change on configuration or form of the metal framework as required. However, it is composed of a linear tubular body as a basic structure, it exhibits poor conformity to a bent portion of the main artery, so that the deformation of the stent and the damage of the blood vessel are liable to be produced with the passage of time or after a long period of time.
(2) Meanwhile, with a stent graft of a metal mesh type which is manufactured to gain improved flexibility, to be sure, it shows good flexibility and improved conformity. However, the problem is that when the stent is diameter-contracted, the total length thereof increases accordingly, thereby making its positioning for placing the same in the diseased part difficult, the stent-placed position moves or drifts after a long period of time following an operation, or the stent is liable to suffer a structure change with the passage of time, such as deformation, etc., caused by the expansion of the stent toward an aneurysm side.
(3) Further, in case of a tubular body stent formed by connecting annular units having the same diameter and the same number of zigzag patterns, there is no problem to be in the form of a straight line so long as it is not bent. When it is bent, however, from the form of a straight line to a curved form, all of central curved portions nearly face or oppose to each other, or central curved portions and inside spaces between curved portions face or opposed to each other, on the inside of curved line R of the stent. Therefore, i) when the central curved portions of adjacent annular units of a stent face each other, and when the stent is bent, the central curved portions on the inside of the curved line R of the stent overlap and touch each other, so that the stent comes to have difficulties in keeping flexibility and durability.
ii) on the other hand, when the central curved portion of an annular unit of a stent and the inside space of the curved portion of an adjacent unit of the stent face each other, and when the stent is curved, the connection portion and the generally straight line portion are not aligned in generally straight line, so that the stent is liable to be kinked when the annular units connected in the form of a curve are contracted or are restored from contraction, and the stent is liable to be deformed with the passage of time.
Thus the prior stent formed by arranging annular units composed of zigzag patterns each and connected in a linear structure is, in principle, not suitable for bending to make a curved form as described above.
(4) There may be employed a constitution in which the connection portion between the annular units is so arranged to have a larger length than the generally straight line portion that the zigzag patterns do not overlap and touch. However, in this constitution, undesirably, the connection portion may be kinked or suffer torture after a long period of time, or the above constitution may result in insufficient expansion force of the connection portion against a diseased part.
In the light of importance of the above problems of the conventional stent, etc., particularly, for providing a stent that is remarkably improved in safety and conformity during delivery within an artery, or the like, and with the performance of stable anchoring or placing at a diseased part for a long period time, the present inventors have made diligent studies and as a result have arrived at the present invention.